1. Field of the Invention
The present invention relates to a non-volatile memory device widely used for equipment such as a computer, a memory card, and a word processor, and a method for producing the same. In particular, the present invention relates to a non-volatile memory device having a high density and a large storage capacity and being capable of electrically writing and reading data and a method for producing the same.
2. Description of the Related Art
There is an example of the above-mentioned non-volatile memory device in which recorded data can be rewritten. The following four kinds are representative of such a device.
(1) A magnetic tape PA0 (2) A magnetic disk PA0 (3) IC non-volatile memories such as an erasable and programmable read only memory (EPROM), an electrically erasable and programmable read only memory (EEPROM), etc. PA0 (4) A magneto-optical disk
Hereinafter, the features of each device will be described.
Magnetic tape
A magnetic tape is the most typical rewritable non-volatile memory device. The magnetic tape has been used as an audio tape, a video tape, etc., because of its low cost. In addition, the magnetic tape has a large storage capacity, so that it can be used as a backup memory for a computer.
However, the magnetic tape has the disadvantage of a long access time required for writing and reading data, since data is written and read in a sequential manner, making it impossible to perform a random access.
Magnetic disk
A magnetic disk is used as an external memory device for a computer, a word processor, etc. In general, the magnetic disk is classified into a floppy disk and a hard disk. The floppy disk is easy to handle and costs low, while the hard disk is difficult to handle and costs high. These magnetic disks have the advantages in that a high-speed random access can be performed and data is relatively easily written and read.
The storage capacity of one 3.5-inch floppy disk is about 1 megabyte, and that of one 3.5-inch hard disk is about 40 megabytes. Thus, there is a limit to the storage capacity and density of the magnetic disk.
IC non-volatile memorex
An IC non-volatile memory is capable of recording data with a high density. Examples of the IC non-volatile memory include an EPROM and an EEPROM. In the EPROM, data is electrically written end erased by using UV-rays, while in the EEPROM, data is electrically written and erased. These non-volatile memories have the advantages of a small and light-weight configuration, a short access time, small consumption of electric power, etc.
Hereinafter, the EEPROM will be described in detail.
FIG. 8 is a cross-sectional view showing an example of the EEPROM. In this EEPROM, a source region 8 and a drain region 6 are formed in an upper portion of a silicon substrate 7. Between the source region 8 and the drain region 6, a channel region 9 is formed. A gate oxide film 5 is formed on the silicon substrate 7 provided with the drain region 6 and the source region 8. On the gate oxide film 5, a floating gate 4 and a control gate 2 are formed in this order. Carriers are stored in the floating gate 4. The control gate 2 controls the injection of the carriers into the floating gate 4. The control gate 2 and the floating gate 4 are insulated from each other by an insulating film 3 such as a silicon oxide film. Furthermore, a surface protection film 1 made of a silicon oxide film, a silicon nitride film, etc., is formed over an entire surface the layered structure.
In the case where data is recorded (i.e., written) in the EEPROM with the above-mentioned structure, a voltage is applied across the drain region 6 and the control gate 2 to allow hot electrons as carriers to be injected into the floating gate 4 through the gate oxide film 5. In the case where the recorded data is erased, a voltage is applied across the source region 8 and the control gate 2 to remove the carriers stored in the floating gate 4, utilizing a Fowlef-Nordheim (N-F) Tunneling phenomenon. The reproduction (i.e., read) of the recorded data is performed by judging between ON and OFF based on a threshold voltage of an inversion voltage in the channel region 9 formed between the source region 8 and the drain region 6.
In the EEPROM, the carriers are injected and removed through the gate oxide film 5, so that the quality and thickness of the gate oxide film 5 are very important factors. For example, in the EEPROM having a storage capacity of 1M bits, the gate oxide film 5 generally has a thickness of about 200 .ANG.. Therefore, the regulation of the quality and thickness thereof are difficult. This causes problems of a high cost due to the decrease in yield. Furthermore., the size of a chip is generally about 7 to 10 mm in both short and long sides. If the chip is provided with a large area in order to increase the storage capacity, the yield is decreased, causing a high cost.
For the reasons described above, in recent years, the storage capacity of the EEPROM is about 1 to 4M bits. Thus, the EEPROM has a smaller storage capacity, compared with magneto-optical disks, magnetic disks, etc., which are other kinds of non-volatile memory devices.
Magneto-optical disk
A magneto-optical disk is one of optical disks, which is a typical non-volatile memory device with a large capacity.
FIG. 9 shows an example of the magneto-optical disk. This magneto-optical disk uses, as a record medium, magnetic thin films 15 and 16 having a vertical magnetization characteristic. For recording data, in a weak magnetic field in the opposite direction to the magnetizing direction of the magnetic thin films 15 and 16, a laser beam 20 is condensed at a condensing region 21 in the disk so as to heat this region. In this way, the data is recorded in the magnetic thin films 15 and 16. On the other hand, for reproducing the recorded data, the Kerr effect or Faraday effect are used. That is, when the laser beam 20 which is linearly polarized is irradiated onto the disk, a transmitted light or a reflected light has its polarization plane rotated in accordance with the magnetizing state of the magnetic thin films 15 and 16. The rotation of the polarization plane is converted into a power light signal by an analyzer and the light signal is detected as an electric signal by an photodetector, whereby the data is reproduced
The above-mentioned magneto-optical disk has been put to practical use as a memory device with a large storage capacity for filing documents, filing images, etc.
Such a magneto-optical disk uses the laser beam 20, so that data is recorded or reproduced in the storage medium through a transparent glass substrate 12 without a member for emitting the laser beam 20 touching the storage medium. Thus, the dirt on a recording side 23 is negligible. In addition, the beam diameter of the laser beam 20 on a substrate side 22 is about hundreds of .mu.m due to the out-of-focus thereon, so that some dirts on the substrate side 22 are not likely to have adverse effects on the recording of data.
Moreover, since the magneto-optical disk records and reproduces data by using the condensed laser beam 20, mass storage recording with a high density is made possible. For example, about 120 megabytes of data can be stored in one 3.5-inch disk.
However, the magneto-optical disk has the disadvantages of large peripheral equipment and a high cost, since a laser, a magnet, a rotary mechanism, etc. are required for writing and reading data.
The problems of the non-volatile memory devices having the above-mentioned features are summarized as follows:
(1) Difficulty in mass storage recording with a high density
The storage capacity of a 3.5-inch floppy disk is about 1 megabyte. Therefore, mass storage recording with a high density is difficult to perform in the floppy disk.
As for IC non-volatile memories such as an EPROM, an EEPROM, etc., high density can be achieved, but a chip area cannot be increased in view of the yield. Thus, mass storage recording is difficult to perform in the IC non-volatile memories.
(2) Weakness for impact and vibration
In a hard disk, a plurality of disks are integrated in order to achieve mass storage recording, and the distance between a magnetic head and a disk corresponding to the magnetic head is set at 1 .mu.m or less for the purpose of achieving a high density. Because of this structure, the hard disk is likely to break down due to impact and vibration. In addition, the hard disk is likely to break down even due to minute dirts adhering to the magnetic head or any of the integrated disks.
(3) Large-sized, complicated, and cost-consuming peripheral equipment for writing and reading data
In either case of a floppy disk, a hard disk, or a magneto-optical disk, data is written and read while a disk is rotated. This makes it necessary to provide a rotary mechanism such as a motor. For this reason, the peripheral equipment is large and complicated.
Particularly in the hard disk, the distance between the magnetic head and the corresponding disk needs to be regulated with good precision and a buffer member is provided for keeping impact-resistance. Therefore, there is a problem that the hard disk tends to be large as a whole and heavy.
As for the magneto-optical disk, a laser and a magnet are used for writing and reading data. This causes the magneto-optical disk to be large, heavy, and cost-consuming.
(4) Long access time for writing and reading data
In either case of a floppy disk, a hard disk, and a magneto-optical disk, there is a limit to the increase in a read speed, since data to be accessed is searched for while a disk is rotated. As for a magnetic tape, a write speed and a read speed are both low.
In order to overcome the problems (1) to (4), the applicant of the present invention has proposed a non-volatile memory using polymer liquid crystal in Japanese Patent Application Nos. 3-138027 and 3-285136. Hereinafter, the outline and problems thereof will be described.
FIGS. 10 and 11 respectively show the structure of the above-mentioned non-volatile memory. The non-volatile memory shown in these figures include two substrates 52 and 55 with a liquid crystal layer 53 sandwiched therebetween. The substrates 52 and 55 are provided with electrodes in a matrix, by which data is written and read.
The substrate 55 is made of silicon. Over the entire surface of the substrate 55, a field insulating film 57 is formed. A plurality of lower electrodes 42 are arranged in a B--B direction (i.e., row direction) on the substrate 55 provided with the field insulating film 57. A plurality of upper electrodes 41 are arranged in an orthogonal direction to the B--B direction (i.e., column direction) above the lower electrodes 42. Between the respective adjacent lower electrodes 42, and between the respective adjacent upper electrodes 41, an inter-electrode insulating film 54 is formed. An heat generating layer 44 is provided between the upper electrodes 41 and the lower electrodes 42. Each intersection of the respective upper electrodes 41 and lower electrodes 42 corresponds to a memory cell 43. The liquid crystal layer 53 is provided with an orientation film 56 on the side of the substrate 55, as shown in FIG. 11. The other substrate 52 is made of glass, The substrate 52 is provided with a counter electrode 51. On the counter electrode 51, an orientation film 58 is formed.
The substrates 52 and 55 are attached to each other and the liquid crystal layer 53 made of polymer nematic liquid crystal is sealed therebetween, thereby obtaining the memory cells 43 as shown in FIG. 11.
In the non-volatile memory thus obtained, data is written by applying an AC voltage to the heat generating layer 44 to heat the liquid crystal layer 53. When the supply of the AC voltage is stopped after heating, the liquid crystal layer 53 is rapidly cooled to form a poly-domain structure. When the supply of the AC voltage is gradually decreased, the liquid crystal layer 53 is gradually cooled to form a mono-domain structure. In order to form the mono-domain structure in the liquid crystal layer 53, the liquid crystal layer 53 may be cooled while being applied with a voltage.
In the non-volatile memory, data is read by applying an AC voltage across the upper electrode 41 and the counter electrode 51 and then measuring the electric capacitance of the liquid crystal layer 53. More specifically, the liquid crystal layer 53 having a poly-domain structure is different from that having a mono-domain structure in dielectric constant. Thus, the difference of electric capacitance caused due to this difference in dielectric constant is measured to read data.
The above-mentioned non-volatile memory has the following problems because of the use of polymer liquid crystal.
First, since two substrates are used in the non-volatile memory using polymer liquid crystal, it is difficult that a driving circuit for driving the memory is integrally formed in the memory. That is, it is difficult to connect a driving circuit provided on the substrate 52 to a plurality of electrodes for reading and writing data provided in a matrix on the other substrate 55, since those electrodes are large in number and provided at a small distance from each other. In addition, small connecting points cause the connection resistance to increase. Because of this, the driving circuit for compensating the influence of the increased connection resistance becomes complicated. These technical difficulties result in low productivity and high cost.
Second, the liquid crystal layer 53 as a record medium has relatively small thermal conductivity. Therefore, it is difficult to uniformly heat the liquid crystal layer 53 so that all of the desired regions of the liquid crystal layer 53 are thermally changed in phase. This is apparent from the results of a computer simulation showing the analysis of heat diffusion in the memory (see FIG. 12).
As shown in FIG. 12, the temperature of a portion of the liquid crystal layer 53 contacting the surface of the heat generating layer 44 to which a voltage is applied is increased to be 58.degree. C., which is enough for the phase transition of the liquid crystal layer 53. The temperature of a portion of the liquid crystal layer 53 in the vicinity of the substrate 52 is 46.degree. C., which is not enough therefor. If a voltage is applied to the liquid crystal layer 53 under this condition, only the portion of the liquid crystal layer 53 contacting the surface of the heat generating layer 44 has its dielectric constant changed. Thus, the change of dielectric constant (i.e., phase transition) of the whole memory cell is difficult to detect.
This drawback can be overcome by prolonging the heating time of the heat generating layer 44. However, the prolonged heating time causes the region at a high temperature to extend to the periphery thereof, resulting in the phase transition in the adjacent memory cells 43. Thus, a new drawback such as the difficulty in obtaining a memory cell with a high density comes up.
Considering the above, in the non-volatile memory using polymer liquid crystal, the distance between the two substrates 52 and 55 is set to be extremely small and the liquid crystal layer 53 is provided with a small thickness, whereby each memory cell 43 is uniformly heated.
On the other hand, the small distance between the substrates 52 and 55 (e.g., about several .mu.m) makes the distance between the upper electrodes 41 and the counter electrode 51 extremely small. For this reason, the upper electrodes 41 and the counter electrode 51 are likely to be in contact with each other, resulting in frequent production of defective products. Thus, there has been a limit to the decrease in cost. Furthermore, in order for a liquid crystal material with high viscosity such as polymer liquid crystal to be uniformly injected into a small gap between the substrates 52 and 55, the liquid crystal material should be injected into the gap after its viscosity is decreased by heating or the liquid crystal material is injected thereto in a monomer state end thereafter polymerized. However, it is industrially difficult to inject the liquid crystal material at a high temperature. In the case where the liquid crystal material is polymerized after being injected into the gap, it is not likely to obtain a uniformly oriented liquid crystal layer.
Third, at present, limited kinds of polymer liquid crystal materials have been developed. Those developed materials have small anisotropy of dielectric constant. Because of this, the change of a dielectric constant due to the change of orientation from the poly-domain structure to the mono-domain structure or vice versa is small, making it difficult to read data.
Fourth, in the case where polymer liquid crystal is used, it is required that the orientation of liquid crystal molecules is set to a predetermined state. Conventionally, there have been the following two methods for setting the orientation state.
(i) Forming an orientation film on a substrate
An orientation film is formed on a substrate by coating an organic resin made of polyimide or the like onto a substrate and subjecting the resulting substrate to an orientation treatment such as rubbing or by diagonally vapor-depositing a silicon oxide or the like onto a substrate.
(ii) Applying a voltage to a liquid crystal layer
A liquid crystal material isotropically changes in phase when a temperature thereof goes up. If the temperature is decreased while applying a voltage to a liquid crystal layer made of the liquid crystal material under this condition, liquid crystal molecules therein are oriented in accordance with the direction of an electric field. Thus, the liquid crystal molecules can be oriented to a predetermined state by raising the temperature of the liquid crystal layer and then lowering the temperature while applying a voltage thereto in the course of the production of a device.
However, the above-mentioned two methods have the following problems.
(1) In the method for forming an orientation film, there is a problem regarding the reliability of the orientation film, when used at a high temperature. In addition, the step of forming the orientation film is added to a production process, increasing a cost. Furthermore, in the case where the orientation treatment such as rubbing is performed, static electricity is generated to damage a transistor or the like of a driving circuit used for heating a memory cell.
(2) In the method for applying a voltage to a liquid crystal layer, an orientation film is not necessary, but a high voltage needs to be applied. Moreover, the liquid crystal layer is fixed under the condition that the liquid crystal molecules are oriented. Thus, this method is difficult to realize.